1. Field of the Invention
The present invention relates to a method of and apparatus for driving a three-phase D.C. brushless motor.
2. Description of the Prior Art
Before describing a prior-art system for driving a three-phase D.C. brushless motor, the relationship among the vector of the magnetic field of a stator, the relative angle .theta. of the magnetic field of a rotor and a torque T developing in the rotor will be explained. As shown in a vector diagram of FIG. 7 A of the accompanying drawings, an angle 103 which the vector 102 of a rotor field defines relative to a stator field 101 is denoted by .theta., and a torque 104 which a rotor generates counterclockwise on this occasion is denoted by T. Then, the relationship between the relative angle .theta. and the torque T ordinarily becomes a waveform whose period is 360 degrees as shown in FIG. 7 B. In addition, it is common that the waveform approximates a sine wave as regards a D.C. brushless motor fabricated so as not to degrade the efficiency thereof. It is also well-known that, in a case where the rotor is multipolar, the relative angle .theta. differs from the mechanical rotational angle of the rotor. In the ensuing description, accordingly, angles concerning the rotor shall indicate electrical rotational angles, and cases where mechanical rotational angles are meant will be clearly stated as such.
FIG. 8 schematically shows the connection of a conventional thee-phase D.C. brushless motor and a prior-art bipolar drive circuit which drives the three-phase D.C. brushless motor while controlling current. As illustrated in FIG. 8, this thee-phase D.C. brushless motor includes a rotor 217 and three stator windings 201, 202 and 203 having phase differences of 120 degrees between the respectively adjacent ones. In addition, three magnetic sensors 220 are arranged at intervals of 60 degrees so as to detect the rotational angles of the rotor 217 at steps of 60 degrees. A group of electronic switches 240 are disposed in association with the stator windings 201, 202 and 203 and the magnetic sensors 220. Each of the electronic switches is turned "on" (is closed) when a logic signal applied thereto is "1", and it is turned "off" (is opened) when a logic signal applied thereto is "0". Usually, these electronic switches are constructed of power transistors etc. The prior-art bipolar drive system turns "on" and "off" the electronic switches, thereby to select two of the three stator windings in succession and to cause currents to flow through only the two stator windings. Such ways of causing currents to flow number six in total because there are three combinations of the two stator windings to be selected, the direction of the current being reversible for each of the combinations, and these six ways generate six sorts of magnetic fields defining angles of 60 degrees between the adjacent ones as shown in FIG. 9(A), respectively. Accordingly, a revolving magnetic field at steps of 60 degrees can be formed by turning"on" and "off" the group of electronic switches to change-over the currents of the stator windings in succession. The motor is rotated by forming the revolving magnetic field in accordance with the rotation of the rotor. That is, a logic circuit 218 decides the positions of the rotor at the steps of 60 degrees on the basis of the output signals of the three magnetic sensors 220 and turns "on" and "off" in accordance with the decided positions, whereby the revolving magnetic field is formed at the steps of 60 degrees. This signifies that the currents are caused to flow through the stator windings in successive change-over fashion so as to generate magnetic fields with which the most effective torques are obtained at any angular positions of the rotor. To provide a better understanding of the logical operation of FIG. 8, reference is made to Table 2 which shows a truth table of logic circuit 218. The prior-art drive circuit is furnished with a current control circuit or voltage control circuit 219, which produces a current or voltage proportional to an instructed voltage 250. It is common to employ the current control circuit 219 in a case where the torque is to be controlled irrespective of the rotational frequency of the motor, and to employ the voltage control circuit 219 in a case where the rotational frequency is to be controlled irrespective of the load of the motor.
FIG. 10 shows the relationship of the torque T to the rotational angle .theta. of the rotor in the case of adopting the prior-art drive method, as stated above. The waveform of the torque T is indicated by a solid line, and the fluctuation width of the torque is indicated by dT.sub.0. Assuming that the torque waveform in FIG. 10 is a sine wave, the torque fluctuation width dT.sub.0 becomes: EQU dT.sub.0 =1-sin 60.degree.=approximately 0.134
and the torque T drops about 13.4% with respect to the maximum value thereof. Moreover, this corresponds to an assumed case where no error is involved in the detection of the angular positions of the rotor, and in actuality, an angular error a.degree. develops in the detection of the angular positions of the rotor due to, e.g., the mounting errors of the magnetic sensors or the like rotor position detector elements. FIG. 11 illustrates this situation, and a torque fluctuation width dT.sub.a in this case becomes still greater as compared with dT.sub.0. That is, assuming that the torque waveform in FIG. 11 is a sine wave, the torque fluctuation width dT.sub.a becomes: EQU dT.sub.a =1-sin (60-a).degree.
By way of example, at a =3.degree., dT.sub.a =approximately, 0.161 and the torque T drops about 16.1% with respect to the maximum value thereof.
The above description has concerned the static torque fluctuations, but when the motor is being rotated, the transient characteristics of the values of the currents flowing through the inductances of the windings also need to be considered. With the prior-art drive system described alone, current I having flowed through the inductance of a certain winding is rendered null, while at the same time current I is suddenly caused to flow through the inductance through which no current has flowed. In actuality, however, the total value of the currents which flow into the motor undergoes a great change on account of, e.g., the generation of a counter electromotive force which is determined by the time constant of the inductance and the rotational frequency of the motor. Due to this change, a great dynamic torque fluctuation arises besides the aforementioned static torque fluctuations at the time of the switching of phases. In this manner, with the foregoing prior-art system for driving the three-phase D.C. brushless motor, the overall torque fluctuation becomes considerably great. Moreover, on account of the great torque fluctuation, considerably loud noise has sometimes occurred during the rotation of the motor.
As expedients for reducing such a torque fluctuation, it has heretofore been proposed to improve the magnetic circuit of a motor and to improve a driving method. Among the prior-art methods proposed, the method improving the magnetic circuit of the motor includes a method which improves the magnetization pattern of a rotor, a method which improves the configuration of stator windings or the configuration of the cores of the stator windings, and so forth. Any of these methods, however, crushes the vicinity of the maximum value of a torque curve to thereby flatten the torque fluctuation. Since the vicinity of the maximum value of the torque curve is originally a part of raising the efficiency of the motor, crushing this part incurs the disadvantage that the efficiency of the motor is lowered.
On the other hand, the prior-art method of reducing the torque fluctuation through the improvement of the driving method includes a method according to which the correction values of torque fluctuation components corresponding to the rotational angles of a rotor are recorded in a read-only memory beforehand, and corrections are made so as to flatten the torque fluctuation by referring to the recorded values, and so forth. Any of these methods, however, premises the joint use thereof with means for detecting the rotational angles at a high resolution and involves the disadvantage of high cost. Moreover, with the methods, in a region where a torque curve falls, current which enters a motor is relatively increased, thereby to lift up a torque. The region where the torque curve falls is originally a part in which the efficiency of the motor is low. Therefore, when the current is increased here, a copper loss increases relatively, to incur the disadvantage that the efficiency of the motor lowers.
An object of the present invention is to provide a method of and apparatus for driving a three-phase D.C. brushless motor which can eliminate the problems of the prior art as stated above.
Another object of the present invention is to provide a three-phase D.C. brushless motor which can eliminate the problems of the prior art as stated above.